The present invention relates to rectifiers and more particularly, to a rectifier having a mesa structure and concave doping profile. The structure of the rectifier results in a reduction of the electric field on the surface thereof and a corresponding improvement in breakdown voltage characteristics.
Semiconductor rectifiers or "diodes" are used in virtually all electronic products to provide directional control of current flow in a circuit. Diodes are fabricated from a P-N junction which, fundamentally, is a structure containing a P-type semiconductor in contact with an N-type semiconductor. In such a device, current will easily flow in the direction from the P-type material to the N-type material. Current does not, however, flow easily in the opposite direction (i.e., from the N-type semiconductor to the P-type semiconductor).
Various methods have been devised to fabricate semiconductor diodes. For example, a P-type semiconductor material can be brought into intimate physical contact with a separate N-type semiconductor material. In practice, however, P-N junctions are usually fabricated by chemically modifying an otherwise uniform crystal to create an abrupt change of doping (i.e., impurity levels) between two portions of the crystal. A semiconductor material frequently used to produce rectifiers is silicon.
Silicon P-N junctions will normally break down at the surface of the device, and not in the bulk silicon as desired, unless special precautions well-known in the art are taken. With higher voltage breakdowns, more care has to be taken to prevent surface breakdown than with lower voltage breakdowns. If sufficient precautions are taken, avalanche or "zener" breakdown will occur in the direction opposite to normal current flow (the "reverse bias" direction), and even large electrical energy pulses in the reverse bias direction will not damage the diode. Silicon diodes operated in the zener breakdown mode are useful in maintaining constant voltages (e.g., power supply output voltages) from several volts to several hundred volts with power ratings up to 50 watts or more.
At high voltages, it is important for rectifiers to have a surface field strength (voltage per unit area) that is at least one-third to one-half the breakdown voltage of the device. It is known that with proper surface contouring, the peak surface field of P-N junctions can be reduced. See, for example, the article entitled "Control Of Electric Field At Surface Of P-N Junctions", R. L. Davies and F. E. Gentry, IEEE Transactions On Electron Devices, July, 1964, pp. 313-323. Practical problems have arisen, however, in producing such P-N junctions. In practice, diodes are typically manufactured using heavily doped P-type and N-type regions with a high ohmic region in between. The width of the high ohmic region is critical in defining the breakdown voltage of the device. However, in previously known structures, including the contoured structures proposed by Davies and Gentry, a tradeoff occurred because as the width of the high ohmic region was decreased, the breakdown voltage at the surface of the device also decreased to unacceptable levels.
It would be advantageous to provide a rectifier having improved voltage breakdown characteristics together with a method for manufacturing such rectifiers wherein precise control of the breakdown voltage is achieved. It would be further advantageous to provide such a rectifier having a surface field that is much lower than the electric field in the bulk semiconductor from which the device is fabricated. Such a rectifier should be easily mass produced and have exceptional high temperature reverse bias stability. It would be further advantageous to provide such a rectifier that is very highly passivated.
The present invention relates to such a rectifier and method for fabricating the same.